Electro-optical arrays are widely used in commercial products. Examples of such products include a phone, a monitor, a television set, and a wristwatch, all of which have pixel arrays used for information display. Further examples include an OLED lighting panel and an OLED luminaire, which have arrays of OLED elements used for illumination.
Display Products
In recent years, there has been a blurring of lines between some of the abovementioned product categories. For example, modern smartphones routinely include cameras and allow viewing of video and television received over wireless networks and the Internet. Additionally, smartphones offer access to many of the same classes of applications (or, “apps” for short) that consumers previously accessed using computers with monitors. These application classes include news, email, instant messaging, games, and office productivity tools. Therefore, within this disclosure, we mean “phone” as commonly understood at present: a relatively small devices with display less than 30 cm in extent, preferably less than 20 cm in extent, more preferably less than 15 cm in extent, and commonly less than 10.2 cm in extent. The term “extent” means the largest transverse dimension along a surface of a display, lighting device, or other electro-optical array. For rectangular displays as are found in common phones and televisions, the extent is the same as the diagonal measure commonly cited as the size of the display. For curved products, “extent” is measured as if the product was laid out flat.
The term “array”, as applied to electro-optical or electroluminescent elements, is understood to refer to a two-dimensional array of such elements formed over a single substrate. A two-dimensional layout of OLED panels, each having a single electroluminescent element would not be considered an array of electroluminescent elements, since each OLED panel has a different substrate from the other OLED panels. The array is considered to be two-dimensional regardless of whether the surface is flat or curved. The surface on the substrate over which such an array is formed is nominally considered to be the top surface of the substrate, regardless of the orientation or curvature of the substrate within a particular product.
It is also useful to define the concept of neighboring elements in such an array. Consider first and second elements of such an array, which have respective first and second centroids. The first and second elements are neighbors if the number of distinct points on the top surface of the substrate that are (a) equidistant from first and second centroid, and (b) farther from the centroids of all other elements of the array, is greater than or equal to two. According to this definition of “neighbor” two adjacent squares on a chessboard are neighbors (all except corner points along their common boundary satisfy both conditions (a) and (b)), two diagonally touching squares on the chessboard are not neighbors (the corner where the squares touch is equidistant from four squares of the chessboard, hence this point does not satisfy condition (b), and no other point meets both conditions (a) and (b) either), and two squares remote from each other on the chessboard are not neighbors (all points satisfying condition (a) are closer to the centroid of some third square than to the first and second centroids).
We use “television” as commonly understood in the art: a relatively large device for playing video-plus-audio programming received from over-the-air broadcast, cable TV, the Internet, wireless network, or by wired transmission from separate nearby equipment such as an optical disk player, a digital video recorder, a computer, or a camera. The display of a television may range from 2 cm to 305 cm in extent, preferably 20 cm to 155 cm, and often 80 cm to 140 cm in extent.
We use “monitor” to mean a display capable of showing changing information over time. Monitors include those found in airport terminals, lobbies of commercial buildings, and kiosks, as well as those associated with a specific computing device such as a tablet, a laptop, or a desktop computer, or otherwise known in the art. “Monitor” may also refers to an information display found in or on a host of embedded systems, ranging from thermostats, refrigerators, automobiles, GPS navigation devices, alarm systems, and many more. Large information display monitors often have an extent from 75 cm to 200 cm, preferably less than 155 cm. Ultra-large information displays are also known. For example, sports stadiums commonly have displays exceeding 100 m2 in area; the stadium exterior display built for the Kazan Universiade measures an astonishing 3700 m2. Of course, these ultra-large displays often comprise a modular array of smaller information display monitors. In such a case, the term “monitor” includes within its scope both the entire stadium display, as well as a single module. In other cases, large information displays are comprised of discrete lamps. A lamp is understood herein to mean a single light-emitting element that cannot be spatially resolved as smaller elements. A lamp is not a monitor, as understood herein. A monitor may be a commercial product by itself, such as a stand-alone monitor for a desktop computer, or it may be part of an integrated system, such as the information display of a tablet computer.
There is a burgeoning class of commercial products known as wearable electronics, many of which incorporate a display. Wristwatches have been common for over one hundred years, and electronic wristwatches have been known for over forty years. Recently, watches with full-color displays have emerged in the marketplace. Other wearable electronic devices with displays include personal music players (such as the Apple iPod™), and head-mount optical displays (such as the Google Glass™). There have been proposals to incorporate wearable electronics into clothing, shoes, jewelry, and other articles of apparel.
All of these commercial products may have displays that are full-color or monochromatic; black and white displays being a special case of monochromatic displays. Displays commonly incorporate individual elements, known as pixels, on a common substrate. Typically, pixels are electrically controlled and are individually controlled, however pixels may be commonly controlled in groups. In an electroluminescent display, such as an OLED display, pixels are individually light-emitting. Other displays have a common light source for multiple pixels, which could be a backlight or edge lighting or ambient light. One common light source may illuminate all the pixels of the display, or merely a group of pixels in a region of the display. In displays with one or more common light source, the individual pixels incorporate electro-optical elements that control the transmission or reflection of light from the one or more common light source. Displays of this type include liquid crystal displays, electrochromic displays, ferro liquid displays, electrophoretic displays, and electrowetting displays. The term “electro-optical element” includes electroluminescent elements such as LED and OLED. Many of these electro-optical elements contain organic materials and have limited tolerance for heat. Many of these electro-optical elements are sensitive to moisture and oxygen. OLED elements are particularly sensitive to moisture, are sensitive to oxygen, and have limited tolerance for heat. While heat tolerance of an OLED varies according to the device architecture and the particular compounds used, 300° C. has been cited as a maximum substrate temperature during an encapsulation process, by Federovskaya in U.S. Patent Application Publication 2009/0081356 A1.
Lighting Products
Electro-optical arrays, in particular electroluminescent arrays, also find use in lighting products. The term “lighting product” refers to any product whose function is to provide illumination of space or objects external to the product. Illumination may be in the visible spectrum or in other portions of the electromagnetic spectrum. OLED panels may be lighting products; OLED lighting panels are commonly organized as an array of commonly controlled but separate light emitting elements on a single substrate. At present, the extent of the array of light emitting elements in an OLED panel may lie within the range from 2 cm to 30 cm, commonly 5 cm to 21 cm, and often 10 cm to 16 cm. In future, as manufacturing technology improves, this array extent may increase to 50 cm, 100 cm, or even larger. In some instances, OLED panels may have light-emitting elements having a plurality of differently colored emissions. For example, ⅓ of the elements may be red, ⅓ green, and ⅓ blue. By varying the relative excitation of red, blue, and green elements, the color and the color temperature of the light may be controlled. Light emitting elements in an OLED panel are commonly organized in rectangular or hexagonal layouts. Although many or all of the light emitting elements in an electroluminescent array of a lighting product are commonly controlled, from the point of view of structure and organizational layout, these light emitting elements are substantially similar to the pixels of a display product. Furthermore, for any given electroluminescent technology, the encapsulation requirements of light emitting elements in display and lighting products are substantially similar. Since encapsulation is of particular interest in this disclosure, it is understood that discussions using the term “pixel” are generally applicable to lighting elements of a lighting product as well, except in those cases where it is clear from the context that the discussion is specific to display products only.
Because OLED panels are at present relatively small, and because designers have exercised their imagination to create complex and artistic structures, many lighting fixtures and luminaires have been conceived as each comprising multiple OLED panels. Such a lighting fixture or luminaire would be a commercial product incorporating a plurality of electro-optical arrays, since each OLED panel itself incorporates an electroluminescent array. A lighting fixture or luminaire is understood to mean a single detachable assembly directly mounted onto a wall, ceiling, floor, furniture, building, frame, pole, tower, truss, or other civil structure, for the purpose of providing illumination. A lighting panel is understood to mean the smallest removable unit from a lighting fixture or luminaire that can be removed and replaced as an integral unit without impairing the capacity of this unit to generate light, in other words, without breaking anything. Although lighting panels and lighting fixtures are often distinct, they can also be the same, for example the common inexpensive plug-in electroluminescent night lights available today. Of course, depending on the electroluminescent technology in use, not all electroluminescent panels will incorporate a two-dimensional array of separate light emitting elements; some technologies may readily allow a panel to be bunt as a single light-emitting element, or alternatively as a one-dimensional array of light-emitting elements.
Flexible Products
Another current trend is toward flexible products. From a manufacturer's standpoint, flexible products are desirable because they can be manufactured at large scale and high volume using a relatively inexpensive roll to roll process, as against the more common discrete manufacturing used today for both display and lighting products. From a designer's standpoint, flexible products are desirable because they can be configured into curved devices, some of which will be rigid curved devices, such as a curved television, while others will be flexible, such as could be integrated into clothing. From a consumer's standpoint, flexible products are desirable because they offer the prospect of lightweight, compact, foldable, and even unbreakable devices.
However, as discussed below, encapsulation suitable for flexible products has not been satisfactorily addressed to date, especially for the stringent encapsulation requirements of OLED elements.
Encapsulation Technology
Materials used in organic light emitting diodes (OLEDs) are well known to be sensitive to oxygen and moisture. Degradation mechanisms are described, for example, by So et al., Advanced Materials, vol. 223, pp. 3762-3777, 2010. As a result, encapsulation is an important part of OLED design. Two main classes of encapsulation are known: (1) use of an encapsulation substrate, i.e. a preformed sheet, and (2) thin film encapsulation.
Encapsulation substrates may commonly be glass or metal, and are commonly spaced from underlying electroluminescent elements with e.g. nitrogen gas fill in between. For example, U.S. Pat. No. 6,111,357 to P. Fleming describes an encapsulation substrate in the form of a glass, metal, or ceramic cover that is attached to an underlying display substrate by a perimeter seal located outside the active area of the display. A metal substrate is opaque and is only suitable for a bottom-emitting display, while a glass substrate is relatively thick and rigid, and not well-suited for roll-to-roll manufacture or flexible displays.
Thin film encapsulation offers manufacturing benefits, but suffers from the relatively high permeability of polymer materials, and the difficulty of depositing or forming thin film layers that are free of pinholes. The permeability requirements for OLED are stringent and limit the choice of suitable materials. One approach to overcoming these problems has been preparation of laminated layers. See, for example, U.S. Pat. No. 4,104,555 to G. Fleming, U.S. Pat. No. 5,811,177 to Shi, and Lewis et al., IEEE Journal of Selected Topics in Quantum Electronics, vol. 10, no. 1, pp. 45-57, 2004. But, the use of laminated layers requires additional process steps, with attendant costs.
Additionally, many variants are known. In U.S. Patent Application Publication 2012/0319141, Kim discloses a combination of a multi-layer thin film seal with a cover attached by a perimeter seal. U.S. Pat. No. 7,368,307 to Cok discloses a flexible substrate attached to a rigid curved encapsulating cover. Neither of these solve the abovementioned problems with encapsulation substrates on one hand, or thin film seals on the other.
It is also known to combine the encapsulant function with other functions. In U.S. Patent Application Publication 2011/0241051, Carter discloses a structured film encapsulant with an integrated microlens array and diffraction grating. This encapsulant is pre-formed, which entails additional manufacturing equipment and cost, and also requires careful alignment between the pre-formed optical structures on the encapsulant and a pixel pattern on an underlying display substrate. Further, Carter's encapsulant is described as comprising an elastomeric polymer (such as polydimethylsiloxane (PDMS)) with one or two coating layers (such as silicon nitride (SiN)). This multi-layer structure involves additional process steps and costs as described above.
A number of authors have been concerned with the separate encapsulation of distinct devices on a mother glass, prior to singulation. U.S. Pat. Nos. 7,091,605, and 7,329,560 both require a perimeter seal around each distinct device, which requires too much space to be workable between neighboring electro-optical elements in a two-dimensional array of elements of a single device. U.S. Pat. No. 6,949,382 to Pichler requires hardening of a planarization layer that substantially covers an entire device, and is fundamentally at odds with encapsulating a flexible device.
Thus, there remains a need for an encapsulation technology that is compatible with roll-to-roll manufacturing, and flexible, unbreakable, or deformable products that incorporate a two-dimensional array of electro-optical elements.